Load-lifting tailgate

ABSTRACT

Mechanism permitting a tailgate structure for opening and closing a truck body to also lift and lower loads between the truck bed level and ground. A parallelogrammatic linkage, powered by means for extending or contracting a diagonal of it, provides essentially level movement from ground to truck bed height when the structure is to function as a load lifting or lowering platform but, upon holding one arm of the linkage, can cause the structure to swing from a relatively horizontal load-platform position at truck bed height to vertical position where the structure functions as a tailgate. An interlock prevents actuation of the mechanism to move the structure to or from its tailgate position unless the said arm is securely held.

United States Patent 3,057,491 10/1962 Schlensker 3,257,011 6/1966Randalletal Primary Examiner-Gerald M. Forlenza Assistant ExaminerRobertJ. Spar Att0rney-- Ely, Golrick & Flynn ABSTRACT: Mechanism permitting atailgate structure for opening and closing a truck body to also lift andlower loads between the truck bed level and ground. A parallelogrammaticlinkage, powered by means for extending or contracting a diagonal of it,provides essentially level movement from ground to truck bed height whenthe structure is to function as a load lifiing or lowering platform but,upon holding one arm of the linkage, can cause the structure to swingfrom a relatively horizontal load-platform position at truck bed heightto vertical position where the structure functions as a tailgate. Aninterlock prevents actuation of the mechanism to move the structure toor from its tailgate position unless the said arm is securely held.

PATENTED M1831 I97! SHEET 2 [IF 6 PATENTED M1831 ism A frond: Ks

PATENTED M1831 Ian PATENTED AUBSI l9?! 3,602,381

SHEET 5 UF 6 IN VEN TORS ,4 7 TOP/V5 Vs LOAD-LIFIING TAILGATE Thisinvention relates to improvements in power-actuated structures adaptedto function as tail gates for closing the rear of truck bodies andwhich, when opened to the level of the truck body bed, may then functionas platforms by which loads to be carries by the trucks may be raised orlowered between the truck bed level and ground level. More particularlythis invention relates to an improved mechanism and its controls for sooperating a tailgate structure.

The truck-equipment art has provided various types of powered tail gateswhich pivot about a horizontal axis so as to function, in a verticalposition, as closures for the ends of endloaded truck bodies and, whenpivoted to a horizontal position, as platforms for lifting loads to andfrom the truck bed level to ground level or to the from loading dockswhich may be at lower levels than the truck beds when the trucks arebacked into position for transfer of loads to and from the docks. Suchtail gates may also function as bridges between loading docks atapproximately the same level as the truck beds where, due to groundlevel conditions, requisite parking angles, or the like, a truck closedby vertically pivoted doors could not be backed up to a loading clock sothat the outer edge of its bed would abut the edge of the loading dock.Such powered tail gates have become increasingly necessary pieces oftruck equipment for cartage and like operations in which loadingfacilities for both pickup and delivery seldom permit loading to andfrom a dock at the same level as the truck bed and'to which dock the endof the truck can be backed for load transfer directly between the dockand truck bed.

Early powered tailgates used by the trucking industry were powered,usually by hydraulic cylinders, only when used as horizontal lifiingplatforms to and from the bed level (hereafter frequently referred tosimply as the lifting operation), such gates being manually hingeablebetween the truck bed level and the vertical position closing the truckbody as a tailgate (hereafter frequently referred to simply as theclosing" operation). Such manually closed gates, particularly as thedemand has developed for heavy tailgates capable of handling heavy loadsduring the lifting operation, have become increasingly impractical andobsolete; this has been due both to the physical strain on the truckdrivers and helpers in moving a heavy gate to a closed position and theunavoidable tendency to such personnel, after unlocking such a closedtailgate, to simply allow it to fall noisily to its bed-level position,with a consequent likelihood of damage to the gate and its liftingmechanism.

Heretofore, the efforts to provide alifting tail gate which is.

power actuated both in its lifting operations and closing operationshave been far from satisfactory. The mechanisms heretofore availablehave been unduly complex, often requiring one or more cylinders or likepowering means for the closing operation separate and apart from thepower means required for the lifting operation. In addition to theinitial expense of such mechanisms, their complexity also made themdifficult to inspect, service, and maintain. Maintenance presents aparticularly serious problem due to the unavoidable exposure ofoperating elements to damage from collision and trafficthrown rocks andstones as well as traffic grime and dirt likely to impair the bearingsand like highly finished surfaces necessary for operation of themechanism.

It is an object and advantage of this invention that it permits poweredoperation both for lifting and closing operations by one set ofhydraulic cylinders operating a mechanism which is especially rugged andinvolves relatively fewer parts and elements than heretofore required.Accordingly, both initial costs and inspection, service, and maintenanceare minimized and simplified even though kinematicsof the linkageachieving selective level lifting and pivotal closing operations iscomplex. The basically rugged simplicity of structure also allows themechanism to be designed for lifting heavy loads while maintaining thesafety and stability of the tailgate as lifting platfonn.

Other objects and advantages of this invention will be apparent from thefollowing specification, claims, and drawings, in which:

FIG. I is a perspective, showing at the end of a truck body and chassispartly broken away, a tailgate operating mechanism made according tothis invention. In this view the gate, per se, is shown in its fullyclosed position with phantom showings of the gate in its lowered andraised positions when it functions as a load-lifting platform.

FIG. 2 is a fragmentary end view, partly in section, of the gateoperating mechanism (less the controls and gate, per se) as shown inFIG. 1, but with the gate lowered to its ground level position.

FIG. 3 is an enlarged detail perspective, partly exploded, of the mainframe and lifting assembly as shown in FIG. 2, particularly showing thecontrol means and, for clarity of illustration, omitting various supportbrackets and, at the right, a truck chassis frame member.

FIG. 4 is a fragmentary side view of the assembled lifting mechanism asshown in FIG. 2, but showing the gate at ground level.

FIG. 4a is a detailed cross section taken along the line 4a-4a of FIG.3, omitting the hydraulic control valve for clarity of illustration.

FIG. 5 is a side view similar to FIG. 4, but showing the position of theelements when the gate is raised to bed level.

FIG. 6 and 6 a are enlarged detail views showing the operation of thelatching mechanism and interlock.

FIG. 7 is a side view, similar to FIGS. 4 and 5, but showing theposition of the elements when the gate is raised to its vertical closingposition.

FIG. 8 is a kinematic analysis of the relative movements of the tailgateas though the gate operating linkage shown in FIGS. 1 to 7 were a trueparallelogram linkage.

Referring to FIG. 1, the tailgate 10 functions as a gate closing theopen end of a truck body B mounted on the truck chassis members C. Thegate is supported in this vertical position and its other positions bymeans of duplicate linkages 11, the outer ends of the arms of each ofthese linkages being secured by pivots 12 and 13 in brackets on theunderside of the gate. As best shown by comparing FIGS. 1 and 4, thepivots Hand 13 are both spaced laterally from each other (to providestability with respect to the actuating hydraulic cylinder includedbetween the arms of each linkage) and act on spaced parallel axes. Thus,it should be noted that the parallel axes of the pivots 12 and 13effectively constitute, by means of their respective brackets integralwith the gate 12, the rearward links RL (see FIG. 2) in theparallelogrammatic operating linkage hereafter more fully described.

The linkages 11 are supported by means of the forward linkage tube 15journaled for pivotal movement in the bearing brackets 14 carried bybracket plates 16 fixed to the main frame tube 17. The main frame tube17 is secured by the frame plates 18 to the truck chassis members C.Except for the incidental upper control rod bracket plates fixed in theangle iron A constituting the rear edge of the truck body bed, the frameplates 18 provide the sole means by which the entire gate and itsassociated operating mechanism are fixed to the truck chassis;accordingly, after the plates 18 have been temporarily secured andpositioned during installation of the gate assembly on a truck, theplates 18 are then usually welded to the chassis frame members C toeffect a relatively permanent mounting of the assembly on the truck. Inthis connection, it should be obvious that in a proper installation thegate is centered to close the rear opening of the truck body. Also, thelinkages 11 are symmetrically located, usually outboard of the plates 18and equidistant from the sides of the gate 10 to provide maximumstability and resist twisting of the gate while it is lifting orlowering a load that may be placed offcenter on the platform surfaceplate 20 of the gate. The channels 19, substantially aligned'with thelinkages 11, which stiffen the platform surface plate 20 are likewisesymmetrically located in thegate (the appearance of a greater spacingbetween the right hand side edge of the gate and its adjacent channel 19than the spacing between the left-hand side edge of the gate and itsadjacent channel, as shown in FIG. 1, is simply due to foreshortening inthis perspective view).

OPERATING LINKAGES The dominant objective of the assembly made accordingto this invention is, or course, to move the gate 10 between itsvertical closing position shown in FIG. 1 to its bed-level portion BLP,as shown in phantom, in order to open and close the gate and to move thegate between its bed-level position BLP and its ground level positionGLP, also shown in phantom, in order to raise and lower loads to becarried by the truck, all by operation of the appropriate control rodlevers 46 and 51. The means by which this objective is accomplished isas follows:

Each linkage 11 is comprised of an arm 21 fixed to the pivotal forwardlinkage tube 15; these two arms, because they carry the main liftingloads imposed on the linkages while raising and lowering the gatebetween the positions BLP and GLP, being hereafter referred to aslifting arms; they are joined by a transverse rear linkage tube 22adjacent their respective pivot pins 13. The thickness and depth ofsection of each of the lifting arms 21 between the linkage tubes 15 and22 are to provide adequate rigidity to the linkages 11 while the gatefunctions as a loading platform.

As best shown in the exploded view of FIG. 3, each linkage 11 includes aforward link fork 24 pivotally mounted as a bearing on the forwardlinkage tube 15 and outboard of the adjacent lifting arm 21, each linkfork 24 being retained on an end of the forward linkage tube 15 by meansof a keeper bolt and washer 23. The outer tine of each forward link fork24 carries a pivot pin 26 on which the forward end of a compression link25 is mounted, the outer ends of the compression links 25 being pivotedon the bracket-mounted pivot pins 12 in gate 10. The distance betweenthe center line of the forward linkage tube 15 and the center line ofeach pivot pin 26 thus constitutes a forward link FL (see FIG. 4) which,together with its substantially parallel effective link RL, constituteone opposed pair of links in a parallelogrammatic gate-operating linkage11, the connecting opposed pair of links in each such linkage being alifting arm 21 and its associated compression link 25. Although eachlifting arm 21 is curved to provide a stiffening depth of section, it isto be noted that it extends between the upper pivot center of the linkfork 24 (also the axis of the linkage tube 15) and the upper pivotcenter provided by the pivot pin 13 and thus, as viewed in FIGS. 4, 5,and 7, effectively constitutes the upper link, with respect to thecompression link 25, of the opposed pair of connecting links betweenlinks RL and FL in each linkage 11.

If a precisely level rise of the gate between the positions GLP and BLPis required, the distance between the axes of the pivots l2 and 13 ismade precisely equal to the distance between the axes of the forwardlinkage tube 15 and a pivot pin 26 and the distance between the axes ofthe tube 15 and a pivot pin 13 is made precisely equal to the distancebetween the axes of pivot pins 12 and 26, whereby a preciseparallelogram linkage is provided. Most users of powered liftingtailgates, however, prefer a so-called ramping action; that is, to aidin loading and unloading at the GLP, the platform surface 20 of the gate10 slopes slightly upwardly (see FIG. 4) and at the BLP, the surfaceslopes slightly downwardly toward the truck bed (see FIG. To achievethis ramping action, one link of an opposed pair in each linkage 11 ismade slightly shorter, as is the link RL in this disclosed embodiment,so that the actual configuration of a linkage 11 is actually slightlytrapezoidal in order to achieve a ramping action. It is to be understoodthat the term parallelogrammatic is intended to be generic to linkagesproviding a ramping action as well as those having a true parallelogramconfiguration to provide a precise level rise of the tailgate in itslifting operation.

In order to power the operation of the linkage l l in moving the gate soas to achieve a substantially level movement between GLP and BLP and apivotal movement between BLP and the closing position, each linkageincludes a double-acting hydraulic cylinder 30 pivotally mounted at itscylinder end between the tines of the link fork 24 and pivotallyconnected at its piston end to an extension of the rearward linkage tube22 outboard of each lifting arm 21. As so positioned, each cylinder 30can exert forces diagonally across its respective parallelogrammaticlinkage 11; because each cylinder 30 is double-acting, the forces itexerts may be either to extend or to contract the diagonal.

To complete the gate operating linkage of this invention (apart from thecontrols) a key and vitalizing element, particularly with respect to themovement of the gate 10 between BLP and the closed position, is alatching mechanism normally operative when the gate is raised to BLP.This mechanism comprises a latch pin 35 carried between upstandingcrankarms 36 integral with the pivoted forward linkage tube 15 and alatching hook 37 pivotally mounted on the main frame tube 17 so as toextend toward the pin 35. The hook 37 is biased toward a latchingposition by a spring 38 and the edge of the hook ahead of its bightconstitutes a cam surface whereby, as the tube 15 is pivoted clockwise(see FIGS. 3 to 7) to move the pin 35 toward the hook 37, the latch pinraises the hook until the pin passes under the point of the hook and islatched in its bight. Thereupon the linkage tube 15 will be lockedagainst the counterclockwise rotation until released from the hook byactivation of the controls and the tube 15, together with its integralcompression arms 21, effectively become fixed members while the tube 15is so locked.

LINKAGE OPERATION The operation of the gate-operating linkage is perhapsmost readily understood if it is first noted that, but for the latchingaction provided by the hook 37, the gate 10 and its associated linkagesl1, irrespective of the amount of extension or com traction of theactuating cylinders 30, would be freely rotatable from the maximumclosed position of the gate shown in FIGS] and 7 to ground level byunrestrained rotation of the forward linkage tube 15 in the bearingbrackets 14. Indeed, with the cylinders 30 fully retracted, thepositioning of the gate 10 at a selected GLP is controlled by adjustingthe stop bolts 39 carried by end brackets on the main frame tube 17. Thestop bolts 39 engage forward edges of the tines of the link forks 24 soas to adjust the vertical portions of the forks 24 and serve as fixedsurfaces which the forks engage while the gate 10 is actuated betweenGLP and BLP (see FIGS. 4 and 5).

Referring to FIG. 4 showing the gate 10 and linkage 11 at GLP, it willbe noted that the cylinder 30 is contracted and, thus, in this positionthe force which it can exert is across the short diagonal of theparallelogrammatic linkage 11, as is also indicated by the dash diagonalline between pivot points 13 and 26in the diagrammatic showing of thelinkage at GLP in FIG. 8. Thus, so long as the effective forward link FLis held against counterclockwise rotation by the stop 39, the diagonalforce exerted upon extending the length of the cylinder 30 byintroducing hydraulic fluid into its cylinder end will force the arms 21and 25 to pivot upwardly so as to provide a greater diagonal lengthbetween the pivot points 13 and 26 until the gate 10 is thereby raisedto its bed level position. See the diagrammatic showing of the linkageat BLP in FIG. 8.

Thus, because the controls to be explained hereafter disactivate thehook 37 when it is desired to lower the gate from BLP to GLP, it shouldbe apparent that the disclosed mechanism, through the hydraulicallypowered lengthening and shorting of the cylinder 30, allows aneffectively level (i.e., disregarding the ramping action) raising andlowering of the gate 10 as a loading platform between a ground levelposition and bed level position and may be stopped and held at anyintermediate position (due to the fact that release of the liftingcontrol lever to a neutral position puts the hydraulic control valve ina neutral position which stops flow of fluid to or from the cylinders40). I

Pivoting the gate from bed level position (where the cylinder 30 isextended) to closed position is achieved by powered contraction of thecylinders 30 without, however, disactivating the hook 37 so as torelease the pin 35 and its associated lifting arm 21 (as occurs'when thegate is lowered from BLP to GLP). Such powered contraction along thediagonal between pivot points 13 and 26 thus pulls the forked links 24containing the forward links FL away from the stops 39; this action,through the compression links 25, pivots the rear links RL about thepivot points 13 until the gate 10 is pivoted from a horizontal positionto a vertical position (see FIG. 7 and the diagrammatic showing of thelinkage at the gate closed position in FIG. 8). Actuation of thecontrols to lengthen the cylinders 30 will correspondingly power thegate at a controlled rate from its closed position to its bed levelposition; indeed, release of the closing control lever to its neutralposition during an opening or closing operation will, as in the releaseof the lifting control lever during a lifting operation,'stop and holdthe gate at an intermediate partly closed position.

CONTROLS In addition to the latching hook 37 carried by the main frametube 17 as described above, this frame tube also carries two othercontrols, best seen in FIG. 3; these are a multipart hydraulic fluidvalve 40 controlled by a reciprocatable valve stem 41 and an electricallimit switch 43 controlled by a toggle linkage 44. These controls 40 and43 (and a safety interlock for the hook 37, to be described hereafter)are directly operated by the rotatable gate closing control rod 45mounted on the main frame tube 17 and extending to an integralgateclosing control lever 46 located at the side of the truck and shownin its neutral position in the several figures of the drawings in whichit appears. Adjacent the valve 40, the rod 45 carries an arm 47 linkedto the valve stem 41. If the lever 46 is swung clockwise, the valve stem41 is pushed in, directing fluid under pressure to the piston rod endsof the cylinders 30, thereby contracting them and causing the gate 10 topivot upwardly to its closed position; if the lever 46 is swungcounterclockwise, the valve stem 41 is pulled out, fluid under pressureis directed to the cylinder base ends of the cylinders 30 to extend themand thereby pivot the gate 10 from its closed position to its bed levelposition; it being understood that during all the foregoing the pin 35is latched by the hook 37. If the pin should not be so latched duringthe above-described operation of the control lever 46, as, for example,if the'gate should be at an'intermediate position between ground leveland bed level, counterclockwise movement of the lever 46, lengtheningthe cylinders 30, would simply lift the gate 10 and clockwise movement,shortening the cylinders 30, would lower it. Because one arm of thetoggle linkage 44 is fixed to the control rod 45, actuation of the lever46 in either direction will close the switch 43 which starts the pumps(not shown) to supply fluid under'pressure to the cylinders.

The upper lifting control rod 50, mounted on the truck body angle iron Aand actuated by its integral control lever 51, operates the valve 40 andswitch 43 by rotating the control rod 50 and also disactivates the hook37 to permit the gate 10 to be lowered from its bed level position toground level, as follows: As best shown in FIGS. 1 and 4, the controlrod 50 carries an arm 52 to which is pivotally connected a connectingrod 53 extending through an eye'53 secured to the upper end of the valveactuating arm 47 (see FIGS. 2 and 3); adjustable stopnuts on theconnecting rod 53 allow its effective length to be adjusted and providefor an adjustable lost-motion connection at the eye 53. Counterclockwisemovement of the lifting operation control lever 51 thus, through the rod53 and arm 47, operates the valve stem 41 to direct pressure fluid tothe piston rod ends of the cylinders 30, shortening them and, thereby,lowering the gate 10 toward the ground level; clockwise rotation of thecontrol lever 51, accordingly, actuates the valve 40 to lengthen thecylinders 30 and thereby raise the gate 10 to its bed level position.

Assuming that the gate 10 is at its bed level position and therebylatched by the hook 37, the hook must be disactivated whenever the gate10 is to be lowered; this isaccomplished by the control lever 51 and rod50 as follows:

As best shown in FIGS. 1 to 3 and 4a, the rod 50 carries a second arm 56pivotally connected to a connecting rod 54 extending into a tube 55which, in turn, is pivotally connected to the hook 37 so that acompression load on the tube 55 pivots the hook 37 against the bias ofits spring 38 to lift the hook off the latch pin 35 and thereby free theforward linkage tube 15 for counterclockwise rotation. An adjustablestop nut on the rod 54 permits transmission of a compression load by therod 54 to the tube 55 while allowing lost motion so that there can be notension load on the rod'54 to restrain counterclockwise movement of thelever 51 to lengthen the cylinders 30 and thereby raise the gate to bedlevel.

So long as the link forks 24 bear against the stops 39, the cylinders 30may be safely shortened tolower the gate 10 toward ground level. But ifthe valve 40 were actuated to shorten the cylinder so as to pivot thegate 10 from bed level to its closed position before the pin 35 wasfully seated and locked in the bight of the hook and, thus, in such apartly latched position, liable to cam the hook upwardly and release thepin 35, such a release while the gate was pivoted above its bed levelwould free the forward linkage tube 15 and allow a free fall of the gate10 and the linkages 1 1. To prevent any such accident and prevent thegate from being pivoted above its bed level, unless the pin 35 is fullylatched, an interlock is provided. As best shown in FIGS. 3, 6, and 6a,the interlock comprises an arm 58 fixed to the end of the control shaft45, this arm having an upper bent position 57 extending toward the hook37.' Cooperating with the bent arm 58 is an outstanding tang 59 fixed tothe adjacent side of the hook. As shown in FIG. 6, if the pin 35 isfully sealed in the bight of the hook 37, the upper bent portion 57 willclear the tang 59 as the control lever 46 is moved-clockwise to actuatethe valve 40 and shorten the cylinders 30. But if the hook 37 should notbe fully seated on the pin 35, the tang 59 will be raised to interferewith the portion 57 and prevent sufficient clockwise movement of thelever 46 to actuate the valve 40. Likewise, as the valve 40 isactuated'by the lever 46, any accidental movement of the control lever51 to simultaneously raise the book 37 will be prevented by engagementof the tang 59 against the underside of the portion 57.

From the foregoing it should be apparent that structures made accordingto this invention may be rugged and withstand hard usage with a minimumof maintenance. In this connection it should be noted that the highlyfinished surfaces of the piston rods are fully withdrawn into theshortened actuating cylinder 30 when the gate is closed for operation intraffic and other coacting surfaces which might otherwise be affected byweathering and traffic grime are similarly unexposed during normal usein traffic. The relatively few number of parts and elements-permitting aclean design and open arrangementallows for ease of inspection andservicing when necessary. The simplicity of the essentially fail-safeoperation permits use by unskilled and inattentive personnel. It is tobe understood, therefore, that this invention is not limited to thespecific embodiments disclosed but may be modified and varies within thescope of the appended claims.

What is claimed is:

1. In a gate for trucks and like load carrying vehicles and itsassociated operating mechanism for effecting a lifting operation on saidgate (raising and lowering loads to and from a relatively horizontal bedlevel position) and a closing operation (moving said gate to and fromsaid bed level position and a relatively vertical position adapted toclose an opening in the vehicle body), the combination comprising a gatestructure, a parallelogrammatic linkage adapted to connect said gate toa vehicle body so as to position said gate relatively horizontally andadjacent the bed of said vehicle body at the uppermost position of saidlifting operation, said linkage including a first link constituting onelink of a pair of opposite links in said linkage and a second linkconstituting one link of the other pair of opposite links in saidlinkage, means to exert forces having components selectively extendingin opposite directions across a diagonal of said linkage, means toresist rotation of said first link during said lifting operations, andmeans to hold said second link when said gate is in the uppermostposition of said lifting operation while said first link is free topivot in a direction opposite to that resisted during said liftingoperation, whereby operation of said means exerting said diagonal forceseffects a closing operation on said gate.

2. A mechanism as defined in claim 1 in which said forceexerting meansis a hydraulic cylinder and the component of forces exerted by itextends across the shorter diagonal of said parallelogrammatic linkagewhen said gate is at its lowermost position in said lifting operation.

3. A mechanism as defined in claim 1 in which the axis of the pivotcommon to said first and second links is substantially parallel to theedge of the vehicle bed at its loading opening and the mounting of saidlinkage to said vehicle body fixes the position of said axis withrespect to said edge.

4. A mechanism as defined in claim 1 in which said link holding means isautomatically actuated when said gate is raised to its uppermostposition during said lifting operation.

5. A mechanism as defined in claim 1 including a first control means foractuating said force exerting means during said lifting operation andwhich disactivates said holding means when said control is actuated tolower said gate during lifting operations.

6. A mechanism as defined in claim 5 including a second control meansfor actuating said force exerting means but which is incapable ofdisactivating said holding means, whereby actuation of said secondcontrol means effects said closing operations.

7. A mechanism as defined in claim 6 including an interlock preventingsaid second control means from commencing said closing operations unlesssaid holding means is actuated.

8. A mechanism as defined in claim l including at least two of saidparallelogrammatic linkages, said linkages being symmetrically locatedand arranged to minimize distortion of said gate and said linkages bythe imposition of an offcenter load on said gate.

9. A mechanism as defined in claim 4 including a first control means foractuating said force exerting means during said lifting operation andwhich disactivates said holding means when said control is actuated tolower said gate during lifting operations.

10. A mechanism as defined in claim 9 including a second control meansfor actuating said force exerting means but which is incapable ofdisactivating said holding means, whereby actuation of said secondcontrol means effects said closing operations.

11. A mechanism as defined in claim 10 including an interlock preventingsaid second control means from commencing said closing operations unlesssaid holding means is actuated.

1. In a gate for trucks and like load carrying vehicles and itsassociated operating mechanism for effecting a lifting operation on saidgate (raising and lowering loads to and from a relatively horizontal bedlevel position) and a closing operation (moving said gate to and fromsaid bed level position and a relatively vertical position adapted toclose an opening in the vehicle body), the combination comprising a gatestructure, a parallelogrammatic linkage adapted to connect said gate toa vehicle body so as to position said gate relatively horizontally andadjacent the bed of said vehicle body at the uppermost position of saidlifting operation, said linkage including a first link constituting onelink of a pair of opposite links in said linkage and a second linkconstituting one link of the other pair of opposite links in saidlinkage, means to exert forces having components selectively extendingin opposite directions across a diagonal of said linkage, means toresist rotation of said first link during said lifting operations, andmeans to hold said second link when said gate is in the uppermostposition of said lifting operation while said first link is free topivot in a direction opposite to that resisted during said liftingoperation, whereby operation of said means exerting said diagonal forceseffects a closing operation on said gate.
 2. A mechanism as defined inclaim 1 in which said force-exerting means is a hydraulic cylinder andthe component of forces exerted by it extends across the shorterdiagonal of said parallelogrammatic linkage when said gate is at itslowermost position in said lifting operation.
 3. A mechanism as definedin claim 1 in which the axis of the pivot common to said first andsecond links is substantially parallel to the edge of the vehicle bed atiTs loading opening and the mounting of said linkage to said vehiclebody fixes the position of said axis with respect to said edge.
 4. Amechanism as defined in claim 1 in which said link holding means isautomatically actuated when said gate is raised to its uppermostposition during said lifting operation.
 5. A mechanism as defined inclaim 1 including a first control means for actuating said forceexerting means during said lifting operation and which disactivates saidholding means when said control is actuated to lower said gate duringlifting operations.
 6. A mechanism as defined in claim 5 including asecond control means for actuating said force exerting means but whichis incapable of disactivating said holding means, whereby actuation ofsaid second control means effects said closing operations.
 7. Amechanism as defined in claim 6 including an interlock preventing saidsecond control means from commencing said closing operations unless saidholding means is actuated.
 8. A mechanism as defined in claim 1including at least two of said parallelogrammatic linkages, saidlinkages being symmetrically located and arranged to minimize distortionof said gate and said linkages by the imposition of an offcenter load onsaid gate.
 9. A mechanism as defined in claim 4 including a firstcontrol means for actuating said force exerting means during saidlifting operation and which disactivates said holding means when saidcontrol is actuated to lower said gate during lifting operations.
 10. Amechanism as defined in claim 9 including a second control means foractuating said force exerting means but which is incapable ofdisactivating said holding means, whereby actuation of said secondcontrol means effects said closing operations.
 11. A mechanism asdefined in claim 10 including an interlock preventing said secondcontrol means from commencing said closing operations unless saidholding means is actuated.